SC1090 false positive: `String(value)` after `typeof value === 'number'` narrowing (union with object arms, 0.0.36 regression)

[somarlyonks]

Summary

String(value) on a union narrowed via typeof value === 'number' inside a for...in loop with a continue guard is rejected:

error SC1090: string conversions of unions with object arms (narrow first: ...) are not supported yet

The value is provably a number at that point. The same code compiled under 0.0.36.

Reproduction

type RelayHeaders = Record<string, string | string[] | number | undefined>

function relayHeaders (source: RelayHeaders, strip: Set<string>): Record<string, string> {
    const headers: Record<string, string> = {}
    for (const name in source) {
        const value = source[name]
        if (value === undefined || strip.has(name.toLowerCase())) continue
        if (typeof value === 'string') {
            headers[name] = value
        } else if (typeof value === 'number') {
            headers[name] = String(value)
        } else {
            headers[name] = value.join(', ')
        }
    }
    return headers
}

console.log(relayHeaders({a: 1, b: ['x', 'y']}, new Set(['c'])))

Actual

repro.ts:10:36 - error SC1090: string conversions of unions with object arms (narrow first: check a discriminant field, or compare with '!== undefined'/'!== null' for unit arms) are not supported yet

Expected

value is narrowed to number in the typeof value === 'number' branch, so String(value) should compile.

Workaround

Avoid the conversion by handling the array arm with Array.isArray and assigning the remaining primitive directly (also lets the value type drop the number arm if the source never carries numbers):

if (Array.isArray(value)) headers[name] = value.join(', ')
else headers[name] = value

Environment

• scriptc 0.1.4
• Node v24.14.0
• macOS (Darwin arm64)